Electronic devices for driving light-emitting semiconductor devices, like light-emitting diodes (LED), often include a current mirror, one end of which is coupled to the light-emitting semiconductor device for determining a current through the light-emitting semiconductor device. The electronic device also includes a control loop for stabilizing the current through the LED at its target value. Another end of the LED is coupled to a power supply, the supply voltage level of which is controlled to a specific level necessary to drive the current through the LED. The LED intensity depends on the LED current. At low supply voltages in the range of the LED forward voltage, the drain voltage of the current mirror output transistor approaches 0 V. Consequently, the current through the LED runs out of control, when the supply voltage at the LED is not high enough to sink the programmed current into the current mirror output transistor. In this situation, the output transistor is typically controlled to have minimum impedance in order to sink maximum current without actually sinking any substantial current. However, in this situation, a very small change of the supply voltage level can cause very high currents to be fed into the transistor. The control loop, in its overdriven state, is unable to counteract these effects. The desired brightness of the LED cannot be achieved, the LED control fails and the electronic device can even be destroyed.
A conventional solution avoids the current overshoot by comparing the drain-source voltage of the current mirror output transistor with a chosen reference value, to turn off the control loop if a the voltage falls below a minimum voltage level in order to avoid the current overshoot. However, there is always a risk that this comparator-based control mechanism may start oscillating around the switching or operating point, and the achievable efficiency is lessened due to the additional margin that has to be preserved to prevent the oscillations.